Paul Jolly Photography Chapter 2

ISO setting

This is an interesting control on a digital camera as this normally controls an amplifier between the sensor and the digital conversion circuits.
As the ISO number is increased the gain of the amplifier is increased making the whites in the image whiter; but not so fast, there is a catch, if we amplify the sensor output we also amplify the noise (we don't get something for nothing here) that is why all the books tell you to use the smallest ISO number for the best shots, normally 100.
Using an ISO of 1600 just means that the image is amplified by 16 times over the ISO 100 version, but the noise is also amplified by the same amount and so your images look 16 times more noisy.
normal and amplified noise